San Francisco Mayor Gavin Newsom's chief of staff, Phil Ginsburg, announced his resignation Thursday, and Newsom named the city's former point man on homeless issues, Trent Rhorer, to the post - the third person to hold the job since the mayor took office in 2004.

Ginsburg, who has been in the post since January 2007, said he is stepping down to spend more time with his family. His resignation is effective Aug. 15.

Newsom said Ginsburg still will play a "key role" in City Hall, but he declined to say in what capacity.

The mayor said that as chief of staff, Rhorer "will get us to the next level of our homeless policy."

Newsom has said he hired Ginsburg partly for his lack of a conventional political resume. During his year-and-a-half on the job, the former human resources manager and union lawyer helped the administration negotiate with labor unions during tough budget deliberations, and last year he was widely praised for helping broker a compromise in a long-running battle over whether to ban cars from Golden Gate Park on Saturdays.

But it was that lack of political experience that also pitted Ginsburg against other administration insiders looking to boost the mayor's political credentials. Rumors about his desire to step down have been swirling around City Hall for months.

"The chief of staff job is to help the mayor steer toward what's in the best interest of the city, (while) those who focus on politics are singularly focused on the mayor's best interest," Ginsburg said Thursday. "And there's often a big overlap in the mayor's best interest and the best interest of the city. There's a natural tension."

Newsom's first chief of staff, Steve Kawa, was a longtime City Hall insider who also worked as deputy chief of staff for former Mayor Willie Brown.

Rhorer, 39, is the executive director of San Francisco's Human Services Agency, where he manages the city's shelter and welfare programs. He also spent four years as Newsom's homeless czar and served as chairman of the Mayor's Interagency Homeless Cabinet, which advises Newsom on homeless policy.

He was instrumental in crafting Newsom's Care Not Cash homeless reform initiative, which helped launch Newsom into the mayor's office.

Rhorer has a bachelor's degree in political science from UCLA and a master's degree in public policy from the Kennedy School of Government at Harvard University. He lives in Oakland and will earn $160,862 annually.

"The top issues that the mayor cares about are homelessness, health care and poverty, and these are issues that I've spent my professional life working on," Rhorer said.